684 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvi. 



hemicerebrums there is a fair-sized fissure occupying the proper loca- 

 tion. The appearances are very suggestive of those in Monachus, 

 where the fissure is rehitively short and undeveloped and sometimes 

 appears interrupted. On the left hemicerebrum where the splenial is 

 interrupted there is a fissure dorsal to, and running parallel with, the 

 callosal portion of the splenial. From its position the name supra- . 

 spl(>nial would be very appropriate, although in some respects it differs 

 from Krueg's. On the right hemicerebrum it is not represented. 



Maniinal gyre. — This, in general, refers to the cortical area l3nng; 

 between the splenial and medilateral fissures, and would therefore i 

 include, in the walrus, the dorsal margin of the hemicerebrum, on i 

 accoiuit of the extended development of the medilateral fissure. 

 What 1 have described as the marginal fissure lies within this gyre. • 

 and the name therefore seems appropriate. 



Collateral fissure. — This fissure appears upon the tentorial surface, • 

 and is perhaps best seen on the ventral or mesal aspect. Its form is 

 somewhat arched, and it lies ventrally to the origin of the splenial. 

 It begins not far from the hippocampal, and its termination caudally 

 is usually more or less closely associated with either the lateral or 

 medilateral fissure, so that in some oases, at least, it may appear upon 

 the ventro-lateral aspect of the hemicerebrum. 



Genual fi,ss\ire. — This is commonly a shallow fissure, lying in front 

 of and is more or less closely associated with the genu of the callosum, 

 from which fact it takes its name. It is found in both hemicerebrums 

 of the walrus. On the left it has a short frontal ])ranch which con- 

 nects superficially with an unnamed minor fissure. On the right hemi- 

 cerebrum it is farther removed from the genu, and ventrally it fuses 

 with the rostral fissure. 



Hostral fissure. — This is also a shallow fissure and lies nearer to the 

 frontal margin of the hemicerebrum. On the left it is a straight fis- 

 sure ;•)() millimeters long. On the right hemicerebrum it is of the 

 same length, and its dorsal end reaches the margin. On account of 

 the convergence of the rostral and genual fissures th(> basal or ventral 

 third represents a combination of these two fissures. 



The lateral ventricle {paracoele). — On removing the dorsal portion of 

 the hemicerebrum just dorsal to the callosum the lateral ventricle is 

 revealed. The cavity dips cephalo-ventrad, ending blindly, to form the 

 precornu; it also dips caudo-latero-ventrad to form the medicornu. 

 The striatum (caudate nucleus) is a convex and well-defined body 

 forming the most of the floor and lateral side of the precornu. Paral- 

 lel with the oblique caudal margin of the striatum is the fimbrial mar- 

 gin of the hippocamp, which, extending down into the medicornu, 

 forms with the hippocamp the floor of this cavitv- Between these 

 two margins (striatum and fimbria) — the rima (great transverse fissure), 

 the choroid (para) plexus — a continuation of the velum enters the floor 

 of the medicornu. The rima is narrow and the thalamus does not 



